We have recognized that Code Division Multiple Access (CDMA) is an efficient technique of using the frequency resource for mobile radio systems. It draws much attention from people in the digital cellular communications area for its large capacity and seamless handoff. In this paper, we define the Multilevel Logic Operation (MLO) which is extended from the conventional binary exclusive OR logic operation. We then apply this MLO to the CDMA Base Station Modulator (BSM) design to reduce the hardware complexity.
A block diagram of a conventional 3-sectorized Base Station Modulator (BSM) which has been developed by Qualcomm Inc. is shown in FIG. 1. In this figure, voice data are encoded by a Vocoder (Voice Coder) and convolutionally encoded via a rate 1/2 convolutional encoder 101 in a channel encoder and scrambler block 100. And then, the coded symbols are interleaved by a block interleaver 102 in order to increase the immunity against burst errors, which frequently occur in a mobile radio environment. These interleaved symbols are scrambled by a long code PN (Pseudorandom Noise) scrambler 103. The scrambled symbols are then pseudorandomly punctured with the cycle of 800 Hz and substituted with power control bit supplied via the microprocessor interface in a puncture control 104 (See, EIA/TIA, IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-mode Wideband Spread Spectrum Cellular System" U.S., July 1993.). The symbol stream is then covered by a Walsh sequence cover 111 within a base band QPSK (Quaternary Phase Shift Keying) modulator block 110, with a distinct Walsh sequence selected via the microprocessor interface. In here, 64-ary Walsh symbols are used and one Walsh code is only assigned to any one user. Forward channels from base station to mobile station are differentiated and identified by Walsh code. Two forward channels transmitting symbol streams are thus formed, corresponding to the two transmit sections. Two forward transmit channels are required to support the softer handoff, which is defined as a handoff occuring among two sectors without temporary disconnection of the call. The Walsh covering guarantees orthogonality between various users in the same CDMA radio channel. After Walsh covering, the symbols are QPSK spreaded in a spreader 114 with two (I and Q channels) PN sequences generated by a Pilot PN generators 112 and 113 respectively. One sequence is used to form an in-phase QPSK channel, and the other PN sequence is used to generate a quadrature-phase channel. Shifted versions of the two basic sequences (which are common to all forward channels) are used to differentiate sectors/cells. After spreading, chip signals are low pass filtered by 48 tab FIR (Finite Impulse Response) filters 115 (See, EIA/TIA, IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-mode Wideband Spread Spectrum Cellular System" U.S., July 1993.). The filtering limits the bandwidth of the transmitted signals, as required to limit interference with adjacent CDMA radio channels. Before filtering, the chip signals are scaled according to their associated rate information. The filters 115 smooth the transitions between symbols with different bit rates. After filtering, the transmit waveforms are scaled to provide power control in the forward channel. The scaling factor is supplied via the microprocessor interface. The BSM provides output to three base station sectors. Each output may be programmed via the processor interface to provide multiplexed I and Q from either of the two transmit sections. The outputs may also be programmed to provide the sum of the two transmit sections as multiplexed I and Q.
There are 192 multiplexed I and Q signals in a 3-sectorized (Alpha, Beta and Gamma sectors) cell. These 64 multiplexed I and Q signals per each sector are combined digitally via digital combiners 130 and converted to analog signals in D/A converters 131. And then, these analog signals are demultiplexed and modulated with the carrier frequency. The modulated I and Q signals are filtered through Low Pass Filters (LPF) 133 and summed before transmitted in summing circuits 134.
As described above, the information bits are spreaded with the PN sequences in the CDMA communications systems. The spreader 114 and long code PN scrambler (103) can be implemented with binary exclusive-OR logic gates since the two inputs of this spreader have binary logic values. However, if the input logic of these gates is not binary, we cannot use these binary logic gates any more. Therefore, extending the binary logic gate to a multilevel logic gate makes digital logic circuit design easier and simpler. Accordingly, to improve the complexity of the digital logic circuit, it is required to define Multilevel Logic Operation, especially multilevel logic exclusive OR operation, extended from the binary exclusive-OR operation and to apply it to the circuit design.